The field of the present invention is four-cycle internal combustion engines having cylinders oblong in cross section.
Generally, in order to obtain high performance in a four-cycle engine, several, sometimes competing, conditions must be satisfied or maximized: (1) The head area of the intake valve or valves and of the exhaust valve or valves is preferably as large as possible in relation to the cross-sectional area of the cylinder bore; (2) The combustion chamber is preferably as compact as possible so that a large portion of the air-fuel mixture is gathered in the vicinity of the spark plug; (3) The spark plug is preferably located to minimize the longest distance from the spark plug to the edge of the combustion chamber as well as have the spark plug as near as possible the majority of the air-fuel mixture; (4) The surface to volume ratio of the combustion chamber is preferably minimized; (5) The compression ratio is preferably fairly high; (6) The chamber configuration is preferably correct for proper mixture flow for intake and exhaust efficiency and for effective mixture distribution. In conventional engines of such construction, the cylinder bore and the piston are circular in cross section so that there is a limit in concurrently satisfying each of the above-stated conditions. If the engine constitutes a multi-valve type by providing two or more intake valves and two or more exhaust valves for each cylinder, for instance, and positioning a spark plug at the center thereof, this arrangement is very advantageous in satisfying certain of the above-stated conditions. However, with an increase in the number of intake valves and exhaust valves, the valve-operating mechanism necessarily becomes complicated, resulting in cost increases which make impractical the adoption of such designs for engines manufactured on a quantity production basis.